Intermediate frequency amplifier



Dec. 12, 1933.

K.POSTHUMUS ETAL INTERMEDIATE FREQUENCY AMPLIFIER Filed Jan. 25, 1932INVENTORS KLAAS POSTHLJM U5 THEOD RUS J WEYERS ATTORNEY Patented Dec.12, 1933 UNITED STATES INTERMEDIATE FREQUENCY AMPLIFIER Klaas Posthumusand Theodorus Josephus Weyers, Eindhoven, Netherlands, assignors toRadio Corporation of America, a corporation of Delaware ApplicationJanuary 25, 1932, Serial No. 588,626, and in France and Belgium June 25,1931.

2 Claims.

Superheterodyne receivers require amplifiers adapted to amplifyselectively a more or less wide band of mean-frequency oscillations. Forthis purpose tuned transformers are usually utilized as couplingelements between the amplifier valves. In order to obtain anamplification characteristic curve which is as rectangular as possible,it is advisable to utilize a band filter in one of the series-branchesof which are connected in series an inductance and a capacity of suchvalues that said branch forms a short-circuit for the mean frequency ofthe band to be transmitted. In this case a drawback consists in thatsaid inductance, which must be adapted to the very high self-inductionof the transformer windings, must be chosen so large as to be almostimpracticable due to which the series condenser becomes so small as tobe almost impracticable.

According to the invention, this drawback is removed by arranging thistuned series branch between a step-down output transformer and a step-upinput transformer. The self-inductions of the secondary and the primarywinding respectively of these two transformers may be comparativelysmall and the self-induction of the series branch may be adapted tothem.

The invention will be more clearly understood by referring to theaccompanying drawing which represents, by way of example, a two-stagemean-frequency, or intermediate frequency amplifier of asuper-heterodyne receiver according to the invention.

In the anode circuit of a mean frequency amplifier tube 1 is connected aprimary winding 2 of an output transformer and in parallel therewith acondenser 3 in such manner that the circuit 2, 3 is tuned to themid-band frequency of the modulation band which is to be selectivelyamplified. This mid-band frequency is, of course, the operatingintermediate frequency. On account of the high internal resistance ofthe tube 1 the self-induction of the coil 2 must have a comparativelylarge value.

The secondary winding 4 of the output transformer has much fewer turnsthan the winding 2 and is tuned by a parallel condenser 5 to the samefrequency as the circuit 2, 3. On the input side of the followingintermediate frequency tube 12 is arranged an input transformer 9, 10whose two windings are tuned by condensers 8 and 11 respectively to theabove-mentioned midband frequency. The primary winding 9 comprisesconsiderably fewer turns than the secondary winding 10. Between the twocircuits 4, 5 and 8, 9 is inserted a tuned series branch 6, 7, theinductance 6 of which is matched to the comparatively smallself-induction of the windings 4 and 9 respectively. Owing to this thecoil 6 and the condenser 'l acquire values lying within limits ofpracticability.

While we have indicated and described one arrangement for carrying ourinvention into effect, it will be apparent to one skilled in the artthat our invention is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdeparting from the scope of our invention as set forthv in the appendedclaims.

What is claimed is: 7

1. An intermediate frequency amplifier comprising a pair of cascadedamplifier tubes, a coupling network connected between the outputelectrodes of the first of said tubes and the input electrodes of thesecond tube, said network consisting of a step down transformer havingits primary coil in the anode circuit of the first tube, a step-uptransformer having its secondary in the grid circuit of the second tube,a condenser connected across each of the windings of said transformersto provide four resonant networks each tuned to said intermediatefrequency, and a series resonant path, tuned to said frequency,connected between points of like potential of the secondary of the stepdown transformer and the primary of the step up transformer, theinductive reactance of said path being chosen to match that of the lastnamed secondary and primary.

2. An intermediate frequency amplifier comprising a pair of cascadedamplifier tubes, 9. coupling network connected between the outputelectrodes of the first of said tubes and the input electrodes of thesecond tube, said network consisting of a step down transformer havingits primary coil in the anode circuit of the first tube, a step uptransformer having its secondary in the grid circuit of the second tube,a condenser connected across each of the windings of said transformersto provide four resonantnetworks each tuned to said intermediatefrequency, and a series resonant path, tuned to said frequency,connected between points of like potential of the secondary of the stepdown transformer and the primary of the step up transformer, theinductive reactances of the last mentioned secondary and primary beingcompara tively small, the inductive reactance of said path being chosento match that of the-last named secondary and primary.

KLAAS POSTHUMUS.

THEODORUS JOSEPHUS WEYERS.

